Comes from the word "chill". If you seperate the beginning and end of the word, you get chi-ll. The "chi" at the beginning is obviously where the chi of 'chi dubs' comes from. Then, you have the "-ll" end of the word "chill" since it is a double l, you just abbreviate this and simply call it dubs, a shortened verious of double. Combine the two and you get Chi-dubs.